The invention has been developed in conjunction with improvement of a pushbroom spectrographic imager. Such an imager is described in Canadian Special Publication of Fisheries and Aquatic Sciences 83: "Analysis of Test and Flight Data from the Fluorescence Line Imager", Dept. of Fisheries and Oceans, Ottawa, 1985.
The aforesaid imager was developed by a group including the present assignee. It was designed for airborne operation, although its application is not limited to that field.
In general, having reference to FIG. 1, the imager comprises:
a transmission grating spectrograph, having an objective lens, a slit assembly, a collimator lens, a reflection diffraction grating, and a camera lens; PA0 a two dimensional charge transfer sensor, such as a Charge Coupled Device ("CCD") integrated circuit array sensor chip, operatively coupled with the spectrograph through a horizontal transport register within the chip, to an output amplifier for digitizing the sensor output; and an instrument control unit for operating the spectrograph and sensor and collecting the output from the amplifier. PA0 dividing and designating the rows into groups of adjacent active rows and adjacent discard rows, the majority of the groups each having several (5-100) rows; PA0 summing each group of active rows on-chip, to produce a single active summed row; PA0 digitizing the active summed rows; and PA0 quickly clearing the balance of the rows (the discard rows) from the sensor without digitizing them, preferably by summing each group of discard rows on-chip and transferring the produced discard summed row to the amplifier for clearing without digitization. PA0 By providing antiblooming capability in the CCD; PA0 By summing individual groups of active rows and individual groups of discard rows in the antiblooming section, to create a lesser number of summed rows in the storage area; PA0 By transporting the active summed rows to the output amplifier and digitizing them to produce output suitable for feeding to the data recording means; and PA0 By clearing the discard summed rows without digitizing them;
The spectrograph functions to focus the light from the scene being viewed onto the slit and to disperse and reimage the light from each point in the line image formed on the slit. Therefore the output of the spectrograph presents a series of line images displaced orthogonally from one another, each line representing a single distinct wavelength of light.
In use, the imager is flown over a narrow, elongated strip of terrain or a "scene" that is to be imaged. The sensor is adapted to be momentarily exposed to light reflected from a discrete narrow strip or "swath" (say 4 m.times.1500 m), usually extending along a line transverse to the direction of flight. The sensor is sequentially exposed to the reflected light emanating from one swath after another. The charges generated in the sensor are read out, digitized and recorded to yield the desired information from the scene as a whole.
More particularly, the sensor head is positioned with the objective lens oriented downwards so that the radiation from the strip is imaged onto the spectrograph slit. Reflected light emerging from the slit is collimated and then dispersed by a diffraction grating. The beam is then focused by the camera lens onto the image or "active" area of the CCD chip.
The CCD chip heretofore used in applicant's imager was a thermoelectrically cooled P86520 series frame charge transfer device manufactured by EEV Inc. (Chelmsford, U.K.). The chip active area comprises a rectangular pattern in rows and columns of pixels (each 15.times.22 micrometers). The pixels are light sensitive silicon. The CCD chip active area is oriented to obtain 578 pixels of spatial resolution across the flight path. The spectrum is dispersed along the columns of the CCD active area, to provide 288 spectral resolution elements, each 1.8 nm wide and covering the spectral range from 430-870 nm. The format of the CCD chip is illustrated in FIG. 2.
In summary then, when integrated with the spectrograph, each row of pixels generates charges indicative of the intensity of radiation having a particular wavelength, said radiation being reflected from a linear array of terrain elements forming a swath of the scene being imaged. And each column of pixels generates charges indicative of the spectrum of an individual element.
A further 290 rows and 578 columns of pixels are provided on the chip and constitute a storage area that is shielded from the light. The columns and rows of the storage area extend on with the same pattern from one end of the active area.
The chip further comprises a horizontal transport register located at the far end of the storage area.
With such chips, the charges of each row of pixels can be transferred to a neighbouring row of pixels by application of external control signals. Stated otherwise, the rows of charges present in the active area pixels may be sequentially advanced through the storage area and transferred, one row at a time, by the register to the output amplifier, for reading and conversion into digitized output or for disposal.
Now, in order to achieve a given resolution from a moving aircraft employing the imaging spectrograph, it is necessary to read out and store all of the charge data from a given exposure in the time it takes for the aircraft to move forward by an amount equal to the distance (the width of the swath) to be resolved on the ground. For example, if the aircraft ground speed is 100 meters per second and the desired ground resolution is 2 meters, then the entire readout has to be completed in 1/50 second.
Since all of the charge has to be cleared from the CCD chip after each exposure, the previous technique of shifting out the individual charges one row at a time, consistent with the transfer rate of the horizontal transports register, results in excessively long readout times. For example, a typical CCD with more than 200,000 pixels, if read out at one microsecond per pixel, would require 1/5 second to digitize all the pixels.
With this background in mind, there is therefore a need for an improved system for reading out the chip to increase the rate with which it processes the charges arising from one exposure, so that good resolution can be achieved even when advancing at typical aircraft speed.